A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is desirable that when a substrate is first loaded onto a substrate holder in preparation for exposure it is held freely so that any stresses can be released. During the loading process, the substrate is supported by so-called e-pins which hold it at three positions. Therefore, the weight of the substrate causes it to distort and it is desirable that this distortion be released before exposures. On the other hand, it is desirable that the substrate be held very firmly during exposure. There are two reasons for this. Firstly, the substrate is subjected to very large accelerations during an exposure sequence in order to achieve a high throughput and must not move on the substrate holder. Secondly, the substrate absorbs energy from the projection beam during exposure and therefore heats up locally. Such local heating can cause thermal expansion and distortion of the substrate. By holding the substrate firmly to the substrate holder such distortion can be resisted.
A substrate holder conventionally has a plurality of burls to support the substrate. The total area of the burls that contacts the substrate is small compared to the total area of a substrate. Therefore, the chance that a contaminant particle randomly located on the surface of the substrate or the substrate holder is trapped between a burl and the substrate is small. Also, in manufacture of the substrate holder, the tops of the burls can be made more accurately coplanar, than a large surface can be made accurately flat.
The substrate is conventionally clamped to the substrate holder during exposures. Two clamping techniques are commonly used. In vacuum-clamping a pressure differential across the substrate is established, e.g., by connecting the space between the substrate holder and the substrate to an under-pressure that is lower than a higher pressure above the substrate. The pressure difference gives rise to a force holding the substrate to the substrate holder. In electrostatic clamping, electrostatic forces are used to exert a force between the substrate and the substrate holder. Several different arrangements are known to achieve this. In one arrangement a first electrode is provide on the lower surface of the substrate and a second electrode on the upper surface of the substrate holder. A potential difference is established between the first and second electrodes. In another arrangement two semi-circular electrodes are provided on the substrate holder and a conductive layer is provided on the substrate. A potential difference is applied between the two semi-circular electrodes so that the two semi-circular electrodes and the conductive layer on the substrate act like two capacitors in series.